This disclosure is directed to a safety relief valve which finds application in relieving increased pressure in a tank or the like. It features a valve member which is directly acted on by a spring and which is also pressure operated so that it snaps open at a set pressure and stays fully open until blowdown pressure is achieved by a pressure drop of the fluid being vented. It does not partially close or throttle down while open. Closure occurs with snap action.
Closure is accomplished with a movable valve member having a soft closure member (a seal ring). The movable valve member and its seal ring is urged toward a valve seat while being accelerated to the closed position to provide snap closing action. An adjustable force is provided by a spring in combination with adjustable back pressure of a bonnet disposed above the valve member to control valve member closure. The safety valve therefore snaps open, stays open fully during venting, and snaps closed when blowdown pressure is achieved.